1. Description of the Related Art
The present invention relates to an image forming apparatus typified by a copier or a laser printer, and particularly to an image forming apparatus comprising developing means for supplying a developer to a photoconductor holding an electrostatic latent image using plural developer supporters and forming a toner image on the photoconductor.
2. Description of the Related Art
In an image forming apparatus, a photoconductor is charged and image exposure according to image data is performed and an electric charge distribution, namely an electrostatic latent image, corresponding to an image pattern is formed on the photoconductor. The electrostatic latent image appears as a visible toner image first by developing toner according to the electric charge distribution. Thereafter, the toner image is transferred to a record material such as paper and is fixed on the record material by thermal fixing.
Here, a development method is performed using a two-component developer in which toner, which is coloring particles, of a resin powder with a particle size of the order of 10 xcexcm and a carrier, which is magnetic particles such as ferrite, magnetite or an iron powder with a particle size of 50 to 150 xcexcm are mixed.
Also, in a developing device, a magnet is provided and a developer is carried in a development part, which is a gap, between the photoconductor and a developing roll by a developer supporter (hereinafter called xe2x80x9cdeveloping rollxe2x80x9d) whose external cylinder rotates. As the developer is carried in the development part, toner adheres on the photoconductor according to a surface electric charge distribution by an electric field determined by a relation between a development bias voltage applied to the developing roll and a surface potential distribution determined by the surface electric charge distribution of the photoconductor.
In this case, a method of using a development bias voltage in which an AC voltage is superimposed on a DC voltage, as a development bias voltage, is described in, for example, JP-B-63-25350 or JP-B-3-2304.
An effect of the development bias voltage using this AC voltage is that even when the DC voltage is low, the amount of toner supplied can be increased by superimposing AC in comparison with the case of only DC. As a result of this, the effect occurs largely in a case making use of in non-contact development in a method of making development without bringing the developer into contact with the photoconductor is performed.
However, a case using the AC voltage in the development bias voltage, when a frequency of the AC voltage is indicated by f and a peripheral speed of the photoconductor is indicated by vp, streaky unevenness in density perpendicular to a rotation direction of the photoconductor occurs at cycle intervals P of vp/f in a rotation direction of the photoconductor as shown in FIG. 2. This is because the AC voltage is superimposed on the DC voltage as a development bias voltage, so that a surface potential of the photoconductor and a development bias voltage of the developing roll change with time and toner is developed more on the photoconductor when the potential difference becomes large and toner is developed less when the potential difference becomes small. For this reason, in the case of using the AC voltage in the development bias voltage, it is constructed so that the streaky unevenness in density does not occur conspicuously by setting the frequency f to a value higher than a certain extent.
However, since the cycle interval of this streaky unevenness in density has been determined by a ratio of a peripheral speed of the photoconductor to a frequency of the AC bias voltage, it is found that the frequency needs to be increased in an image forming apparatus with a large peripheral speed of the photoconductor, namely a high printing speed. But, when the frequency of the AC voltage is too high, the toner cannot follow a change in direction of an electric field caused by the AC voltage and an applied effect of an AC bias is not taken. From this, it has been determined that a frequency of 10 kHz or lower, particularly a range from 1 kHz to 5 kHz is effective as a frequency of the AC voltage.
As a result of this, in an image forming apparatus with a high peripheral speed, an occurrence of the streaky unevenness in density cannot be controlled even when the action of the AC bias voltage is in the effective frequency range.
An object of the invention is to provide an image forming apparatus in which streaky unevenness in density does not occur in a direction perpendicular to a rotation direction of a photoconductor as described above when an AC voltage is used as a development bias voltage in a developing device equipped with plural developer supporters.
The object is achieved by an image forming apparatus comprising a photoconductor supported endlessly movably, latent image forming means for recording and forming an electrostatic latent image on the photoconductor, and developing means for supplying a developer to the photoconductor holding the electrostatic latent image and forming a toner image on the photoconductor, characterized in that the developing means includes plural developer supporters provided opposite to the photoconductor, development bias applying means for applying a development bias voltage generated by superimposing an AC voltage on a DC voltage to each of the developer supporters, and phase shifting means for shifting a phase of the AC voltage in the development bias voltage in each of the developer supporters.